The present invention relates in general to filter elements, and more particularly to filter elements made from porous sintered powdered metal substrates which have been impregnated with a finely divided metal oxide as filter membranes.
Conventional filter elements are known which have porous metal substrates and membranes or coatings thereon of a finely divided metal oxide, such as a titanium oxide or zirconium oxide. The state of the art is exemplified by the tubular filter elements disclosed in U.S. Pat. No. 4,888,114 issued to Gaddis et al., which is owned by the assignee of this invention. Gaddis et al. disclose the provision of a 316 L stainless steel particulate tubular substrate on which is impregnated a titanium oxide. Gaddis et al. make their tubular support members out of particles of 316 L stainless steel that are approximately 30 microns in size. A slurry of titanium dioxide is forced into an inside wall of the tubular substrate and sintered at 1093xc2x0 C. in a reducing atmosphere such as hydrogen.
A drawback of stainless steel-based tubular filter elements is that they are attacked by acidic, chloride-based environments. A need therefore exists for filter elements with porous metal substrates which can better survive halogenic acid attack, and which can be used more effectively in converted corn syrup and other high-chloride, low-pH environments.
The present invention provides a filter element which uses a porous metal substrate, the metal of the substrate consisting of a nickel-based alloy having at least fifty-six percent nickel, at least sixteen percent chromium and at least five percent molybdenum by weight. A sintered metal oxide powder is supported by this substrate.
Preferably, the nickel-based alloy used as the support further includes at least three percent by weight of tungsten. More preferably the alloy forming the substrate comprises at least twenty percent chromium and at least thirteen percent molybdenum by weight.
A method of fabricating tubular elements according to the invention selects a nickel-based alloy, containing substantial amounts of chromium and molybdenum, in a powder form with a particle size that is no larger than 150 microns and more preferably no larger than 44 microns. A tubular mold is filled with the powdered alloy and is subjected to an isostatic pressure of at least 37,000 pounds per square inch to form an unfired workpiece. The workpiece is fired and subsequently impregnated with a powdered metal oxide, such as zirconium or more preferably titanium oxide. The impregnated workpiece is again fired under a reducing atmosphere at a temperature substantially below the temperature attained in the first firing step. A sintered, metal oxide coated tubular filtration element according to the invention results.
Preferably, titanium oxide forms the filtration membrane. In a preferred embodiment in which a rutile form of titanium dioxide (TiO2) is used as a starting material, and responsive to the step of firing the workpiece under a reducing atmosphere, the applied rutile form is converted to one or more reduced titanium oxides of the formula TixOy, where y is less than 2x. The titanium oxides forming the membrane consist mostly of Ti4O7 with some Ti2O3 and Ti6O11.
Tubular filtration elements manufactured according to the invention exhibit substantially superior resistance to attack by high-chloride, acidic solutions and outperform their stainless steel counterparts.